The present invention is directed towards the control of a stepper motor and, more particularly, to controlling the maximum output torque of the motor to match load torque requirements by varying the motor speed.
There are many mechanical systems where it is required to drive a load where the drive torque required to drive the load is not constant. As an example, the torque required to drive an automobile in the normal manner varies from the time motion is started until driving speed is reached. It is desirable for the engine to deliver more torque to the wheels when the automobile is being accelerated than when moving at a constant or decreasing speed. In the automobile, a transmission is sued to change the mechanical advantage in the drive train to vary the drive torque delivered to the wheels. In a similar manner, the present invention uses the input to a stepper motor to drive the motor at different speeds, depending on the torque required to drive a load, during the drive cycle.
Some examples of stepper motor controls are found in the following prior art references: U.S. Pat. No. 3,909,125 discloses a control circuit for accurately positioning the shaft of a multi-phase stepper motor by selective energization of respective phases of the motor under the influence of a low duty cycle power supply in a one-way clutching mechanism. U.S. Pat. No. 3,921,043 discloses an apparatus for maintaining constant the torque output from a first and second motor coupled to a first and second reel.
More particularly, the present invention is directed towards a mechanical drive system wherein a load is driven through a duty cycle with varying torque requirements. The drive system, including a stepper motor, is adapted to operate at a first speed consistent with a first output torque requirement, and later at a second speed consistent with a second torque requirement.